In order to increase the bandwidth of microwave-based communications systems, it is common practice to employ polarization technologies that effectively double the utilization of a given spectrum. Two basic categories of polarization are linear polarization and circular polarization. Linear polarization contains two orthogonal components, vertical and horizontal. Circularly polarized signals contain both vertical and horizontal components separated by a 90-degree phase difference. Whether or not a circularly polarized signal is considered to be polarized in a right hand sense or a left-hand sense is dependent on which of the two components, vertical and horizontal, leads the other in phase. Since linearly and circularly polarized signals both have vertical and horizontal components, it is not possible to further increase the capacity of communications systems by employing both linear and circular polarization on the same frequency.
In a typical microwave receiver, linearly polarized signals are separated in a waveguide by orthogonally located probes. The signals are then processed independently. In the case of circular polarization, an additional processing step is needed to separate the Right-hand polarized and Left-hand polarized signals. It is common practice in circularly polarized receiving systems to employ a waveguide polarizer, placed between the antenna output and the waveguide section containing the orthogonal probes. This polarizer converts the vertical and horizontal components of a given circularly polarized signal into a single component that emerges either at the vertical probe or the horizontal probe, depending on the sense of circular polarization. It is also possible to effect this conversion of the vertical and horizontal components of circularly polarized signals by means of a 90-degree combiner located after the components have been picked-up by the vertical and horizontal probes. In an embodiment comprising a 90-degree combiner, the antenna output is connected directly to the waveguide section containing the probes, as is the case in linearly polarized systems.
In the use of satellites for the broadcast of, for example, television signals, there are instances where two satellites are collocated in space, and transmitting on different frequencies. In many cases these frequencies are close enough that they can be received by one antenna and one low noise block downconverter (LNB). An example would be the Galaxy satellite, which transmits linearly polarized signals in the band 10.95-11.2 GHz, and the Nimiq satellite, which is collocated with the Galaxy satellite and transmits circularly polarized signals in the band 12.2-12.7 GHz. However, because the two satellites have different polarization schemes, the prior art LNB technologies require two separate and distinct antenna/LNB assemblies, one for linear polarization and one for circular polarization.
For this reason, it can be appreciated that it is desirable to have a single antenna and LNB system, which is capable of simultaneously receiving R.F. signals of both linear and circular polarizations.
It is an object of this invention to provide an apparatus for downconverting both dual polarized linear and dual polarized circular microwave signals in a single antenna/LNB apparatus, and for providing separate outputs for all senses of polarization
It is a further object of this invention to provide an apparatus for switching of all available senses of polarization in order to obtain all such senses of polarization at any one of a multiplicity of outputs.